The production of acrolein and acrylic acid from propylene and the production of methacrolein and methacrylic acid from isobutylene or tertiary butanol by the gas-phase catalytic oxidation reaction using the so-called molybdenum-bismuth composite oxide catalyst, and the production of acrylonitrile or methacrylonitrile by the gas-phase catalytic ammoxidation using the same catalyst are well known. However, when the reaction is continued in a long production run, this catalyst suffers deterioration or degeneration so that the conversion and selectivity of the reaction are lowered with time.
Several technologies are known for controlling the deterioration of the composite oxide catalyst. Japanese Unexamined Patent Publication (Kokai) No. 59-193136, for instance, proposes a method which comprises a molybdenum oxide-bearing substance prepared by supporting molybdenum oxide on an inert heat-resisting inorganic substance, and Japanese Examined Patent Publication (Kokoku) No. 63-38331 proposes a method in which a plurality of molybdenum-bismuth multi-component catalysts prepared with a gradient of activity by varying the kinds and/or amounts of alkali metal and thallium group elements in the molybdenum-bismuth multi-component catalyst are arranged in an increasing order of activity from the reactant gas inlet side to the outlet side of a reactor.
Japanese Examined Patent Publication (Kokoku) No. 53-30688 proposes a method in which the catalyst is diluted with an inert substance and packed in such a manner that the catalyst activity increases either continuously or stepwise to 100% in the direction of flow of the reactant gas, viz. from the inlet to the outlet of a reactor.
However, Japanese Unexamined Patent Publication (Kokai) No. 59-193136 has the disadvantage that the catalyst must be supported on an inert heat-resisting inorganic carrier and, moreover, the selectivity of the desired compound according to the process disclosed therein is not sufficiently high for commercial use.
The technologies disclosed in Japanese Examined Patent Publications (Kokoku) Nos. 63-38331 and 53-30688, which are intended to control the catalyst activity by varying the composition of the catalyst or diluting the catalyst to avoid local temperature rise, are capable of suppressing to some extent the loss of molybdenum from the catalyst which is a cause of catalyst deterioration but do not provide for long-term inhibition of catalyst deterioration. Thus, these prior art technologies are not sufficiently satisfactory for practicing on a commercial scale.
Several methods have also been proposed for enabling the reuse or regeneration of the composite oxide catalyst deteriorated in a prolonged course of reaction.
By way of example, Japanese Unexamined Patent Publication (Kokai) No. 55-67335 describes a method which comprises subjecting a deteriorated composite oxide catalyst to a heat treatment in an inert atmosphere at a temperature between 500.degree. C. and 800.degree. C., and Japanese Unexamined Patent Publication (Kokai) No. 57-56044 discloses a method which comprises subjecting a deteriorated composite oxide catalyst to a heat treatment in a reducing atmosphere at a temperature between 200.degree. C. and 700.degree. C. and then calcining the catalyst in a molecular oxygen-containing atmosphere at a temperature between 550.degree. C. and 700.degree. C.
However, these methods are primarily intended for the treatment of the catalysts used in ammoxidation and, therefore, when the high-temperature calcination procedure disclosed is applied to a catalyst for the partial oxidation of propylene, isobutylene or tertiary butanol, the catalyst tends to be sintered with the resultant decrease in activity.
Referring to the catalyst for use in the partial oxidation of propylene or isobutylene, Japanese Unexamined Patent Publication (Kokai) No. 61-33234 proposes a heat treatment at 380.degree. C.-540.degree. C. in an atmosphere substantially composed of air, and Japanese Unexamined Patent Publication (Kokai) No. 63-137755 discloses a heat treatment at 300.degree. C.-500.degree. C. in the presence of an oxidizing gas containing both molecular oxygen and steam. Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 5-184945 discloses a method in which 2-10 weight % of the deteriorated catalyst is withdrawn from the reactant gas inlet region and the remainder of the deteriorated catalyst is heat-treated at 300.degree.-500.degree. C. in a molecular oxygen-containing gaseous atmosphere for regeneration.
However, these methods for catalyst regeneration by heat treatment are not sufficiently effective, and as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-184945, the severely deteriorated portion of the catalyst in the vicinity of the reactant gas inlet can hardly be regenerated. Moreover, repeated regeneration is impossible.
It is known that the dissipation of molybdenum is a cause of catalyst deterioration and, therefore, it has been proposed to compensate for the loss of molybdenum by certain means for the purpose of regenerating the catalyst. Japanese Unexamined Patent Publication (Kokai) No. 50-49201 describes a regeneration method in which the catalyst is contacted with fluidized particles of a substantially inert molybdenum-containing carrier. Japanese Unexamined Patent Publication (Kokai) No. 52-131989 describes a method for catalyst regeneration in which the deteriorated catalyst is saturated with a solution containing at least molybdenum and bismuth and then calcined, and Japanese Unexamined Patent Publication (Kokai) No. 57-56044 discloses a method which comprises adding a molybdenum compound to the deteriorated composite oxide catalyst, heat-treating the mixture at 200.degree.-700.degree. C. in a reducing atmosphere and further calcining the heat-treated catalyst at 550.degree.-700.degree. C. in a molecular oxygen-containing atmosphere.
However, all of these methods are mainly concerned with the regeneration of the catalyst for the ammoxidation of propylene, which is used at a high temperature. Thus, in the processes described in Japanese Unexamined Patent Publication (Kokai) Nos. 50-49201 and 52-131989, the selectivity of the fixed-bed partial oxidation reaction of propylene, isobutylene or tertiary butanol with the regenerated catalyst is not as high as satisfactory. In the process disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-56044, the regenerated catalyst tends to be sintered to detract from catalytic activity.
An object of this invention is, therefore, to provide a method for inhibiting deterioration of the high-activity catalyst used in the production of an unsaturated aldehyde and unsaturated carboxylic acid by the gas-phase catalytic oxidation of propylene, isobutylene or tertiary butanol, a method for reuse of the so-deteriorated catalyst, and a method of regenerating the catalyst.
This and other objects will be apparent from the following description.